Problem: Tiffany is 16 years older than Christopher. Four years ago, Tiffany was 3 times as old as Christopher. How old is Christopher now?
Solution: We can use the given information to write down two equations that describe the ages of Tiffany and Christopher. Let Tiffany's current age be $t$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $t = c + 16$ Four years ago, Tiffany was $t - 4$ years old, and Christopher was $c - 4$ years old. The information in the second sentence can be expressed in the following equation: $t - 4 = 3(c - 4)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $t$ and substitute it into our second equation. Our first equation is: $t = c + 16$ . Substituting this into our second equation, we get the equation: $(c + 16)$ $-$ $4 = 3(c - 4)$ which combines the information about $c$ from both of our original equations. Simplifying both sides of this equation, we get: $c + 12 = 3 c - 12$ Solving for $c$ , we get: $2 c = 24$ $c = 12$.